This invention relates to a scaling device for dividing an input frequency by a mixed number. It has particular utility as a fullscale setting device for a digital (frequency encoded) output of a flowmeter, such as a magnetic flowmeter.
Fluid flowmeters produce an output signal which is indicative of flow rate. In the case of a magnetic flowmeter, for example, the output signal may be proportional to flow velocity over a wide range of flow. For an incompressible fluid flowing in a filled pipe, the output signal is also proportional to the volume rate of flow over a wide range of flow rates. The output signal of a magnetic flowmeter is generally in the form of a DC (analog) signal, the magnitude (voltage) of which is proportional to flow rate. In the process of calibrating a flowmeter, it is usual to adjust a zero setting, then to adjust the calibration, then to adjust a "span" adjustment to set the output signal at its maximum for a maximum expected flow rate. Setting of the span adjustment is made more difficult because the adjustment is usually a potentiometer, and the fullscale setting for maximum flow rate must be set empirically in the field.
In certain magnetic flowmeters, the analog output signal is converted to a train of pulses, the frequency of which is indicative of flow rate. In these meters, the span adjustment is likewise a potentiometer in the analog portion of the signal processing chain. Additionally, a scaling device may be provided which divides the frequency output by a selectable integer to yield a desired output for a particular pipe size, for example, one pulse per gallon. The scaling device does not affect the full-scale frequency, which is typically 10 KHz, but rather divides the output frequency by an integer to provide a relatively low frequency signal. The scaling device inherently reduces the speed of response and the precision (resolution) of the meter.